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Chapter 3 Reactions Combustion Decomposition Combination Chapter 4 Reactions Exchange reactions (Metathesis) • Formation of a precipitate • Formation of a gas • Formation of a week or non-electrolyte Single Displacement Reactions • Redox reactions • Oxidation numbers Mary J. Bojan Page 1 Chapter 4 Balancing Equations Law of conservation of mass: matter cannot be lost in any chemical reaction Mary J. Bojan Page 2 Chapter 4 Patterns of reactivity Know how to balance a chemical reaction • (Complete) Combustion reactions (Chapter 3) C3H8(g) + 5O2 (g) → 3CO2 (g) + 4H2O(g) • Combination reactions (Chapter 3) 2Mg(s) + O2(l) → 2MgO(s) • Decomposition reactions (Chapter 3) PbCO3(s) →PbO(s) + CO2(g) • Exchange reactions (Chapter 4) Precipitation Pb(NO3)2(aq) + 2KI(aq) → PbI(s) ↓ + 2KNO3(aq) Neutralization NaOH(aq) + HCl(aq) → NaCl(aq) + H2O(l) Gas formation 2HCl(aq) + Na2S(aq) → H2S(g) ↑ + 2NaCl(aq) • Single displacement reactions (Chapter 4) 2Ca(s) + O2(g) → 2CaO(s) Mary J. Bojan Page 3 Chapter 4 Write balanced reactions for The combustion of ethanol CH3CH2OH The decomposition of lead carbonate PbCO3 The combination of nitrogen and hydrogen to form ammonia (NH3) Mary J. Bojan Page 4 Chapter 4 Exchange reaction (Double Displacement) Exchange positive ions to get products AD + XZ → AZ + XD Example: Reactants: Pb(NO3)2 + KI Pb(NO3)2 + KI → PbI2 + KNO3 Balance the reaction. Pb(NO3)2 + KI → PbI2 + KNO3 Does anything happen? Do you have to do the reaction to know? Mary J. Bojan Page 5 Chapter 4 Metathesis Reactions (Double Displacement) Given reactants: exchange positive ions to get products Requires a driving force (otherwise nothing happens) Driving Forces 1. Precipitate forms 2. Weak or nonelectrolyte forms 3. Gas forms How do you know what is happening? Net Ionic Equation Mary J. Bojan Page 6 Chapter 4 Example: Precipitation Reaction Mix silver nitrate and sodium chloride. What happens? 1. Molecular Equation 2. Ionic Equation Spectator ions: 3. Net Ionic Equation Mary J. Bojan Page 7 Chapter 4 SOLUBILITY RULES FOR COMMON IONIC COMPOUNDS IN WATER 1. Almost all ammonium and alkali metal salts are soluble. 2. Most nitrates, acetates, chlorides, bromides, and sulfates are soluble. Exceptions: silver halides sulfates of Ca, Ba, Pb 3. Most sulfides, carbonates, phosphates and hydroxides are insoluble. Exceptions: alkali salts ammonium salts. (See #1 above.) See Table 4.1 of Brown, LeMay, and Bursten for a more comprehensive listing. Mary J. Bojan Page 8 Chapter 4 Formation of Weak or Nonelectrolyte (neutralization reaction) Acid + base → salt + water Molecular Equation Ionic Equation Spectator ions: Net Ionic Equation Mary J. Bojan Page 9 Chapter 4 Examples: What is the net ionic equation for the reaction between sodium hydroxide and acetic acid? What is the net ionic equation for the reaction between sodium chloride and potassium iodide? Mary J. Bojan Page 10 Chapter 4 GAS FORMATION • Direct production of a gas CO2, H2S, NO2, SO2 • Production of weak acid which decomposes. Example: Molecular Equation NaHCO3(aq)+HCl(aq) → H2CO3(aq)+NaCl(aq) ↓ CO2(g)+ H2O(l) Ionic Equation Na+(aq) + HCO3− (aq) + H+(aq) + Cl− (aq) → H2CO3 (aq)+ Na+(aq)+Cl− (aq) ↓ Na+(aq)+Cl− (aq)+CO2(g)+ H2O(l) Net Ionic Equation H+(aq) + HCO3− (aq) → CO2(g)+ H2O(l) Mary J. Bojan Page 11 Chapter 4 Single Displacement reactions (Oxidation-Reduction) Zn(s) + CuSO4(aq) → ZnSO4(aq) +Cu(s) What is oxidized? What is reduced? What is the oxidizing agent? What is the reducing agent? Ionic equation: Net ionic equation Rules for determining Oxidation States Mary J. Bojan Page 12 Chapter 4 1. Oxidation state of atom in elemental form is zero. e.g. Cl2 O2 P4 C(s) S8 2. The oxidation number of a monatomic ion equals its charge. 3. Some elements have “common” oxidation numbers that can be used as reference in determining the oxidation numbers of other atoms in the compound. Alkali metals +1 Alkaline earth metals +2 Fluorine –1 Cl, Br, I almost always –1 O usually –2 peroxides (–1) & superoxides possible H usually +1 Hydrides: metal-H compounds (–1) 4. Sum of oxidation numbers is equal to overall charge of molecule or ion: • For a neutral compound the sum of oxidation numbers equals zero. • For a polyatomic ion, the sum of the oxidation numbers is equal to the charge on the ion. 5. Shared electrons are assigned to the more electronegative atom of the pair: more electronegative atom will have a negative oxidation number. Mary J. Bojan Page 13 Chapter 4 ActivitySeries Table 4.5 Will the metal element displace the atom from its compound in a reaction? Na(s) + H2O(l) → Au(s) + H2O(g) → Zn(s) + AgNO3(aq) → Active metals (groups IA, IIA, Al) very reactive! Inactive (inert metals): jewelry metals Do not react readily Other metals in between Mary J. Bojan Page 14 Chapter 4